The present invention relates to an imaging lens used in electronic imaging devices such as digital still cameras and digital video cameras that use solid-state imaging elements such as CCDs and CMOS. More specifically, the present invention relates to an imaging lens suitable for compact mobile cameras installed in portable telephones, portable information terminals (PDAs), and portable personal computers.
Examples of conventional electronic imaging devices that use solid-state imaging elements such as CCDs include monitoring cameras and video cameras. These devices have been used primarily to capture moving images. For this reason, a high degree of performance was not required for these CCDs and imaging lenses.
In recent years, however, as digital still cameras are becoming popular, needs for high-performance solid-state imaging elements and imaging lenses have increased. As a result, significant technological strides have been made in the field of solid-state imaging elements such as CCDs, which have made it possible to obtain images approaching those of film cameras while also providing compactness and a high pixel count in a compact CCD and the like. Thus, a need for providing imaging lenses that provide high performance while being compact, thin, and less expensive has become stronger.
With solid-state imaging elements such as CCDs, a microlens is disposed on the surface thereof to allow efficient entry of light. If the angle of the light incident to the solid-state imaging element is too large, vignetting takes place, which prevents light from entering into the solid-state imaging element. Thus, it is preferable for the exit pupil of the imaging lens to be provided as far as possible from the imaging plane.
Also, it is necessary to keep a certain length of back focus to provide necessary components between the imaging lens and the solid-state imaging element, e.g., a low-pass filter for preventing color moiree and an IR cut filter for correcting spectral sensitivity of the solid-state imaging element.
Conventionally, many cameras have used triplet-type imaging lenses because these use a small number of lenses to provide relatively good optical properties while easily allowing the design to be made compact (see, for example, Japanese Laid-Open Patent Publication Number Hei 07-168095, Japanese Laid-Open Patent Publication Number Hei 10-48516, Japanese Laid-Open Patent Publication Number Hei 08-234097, Japanese Laid-Open Patent Publication Number Hei 07-325251, and Japanese Laid-Open Patent Publication Number Hei 11-52227). With these triplet-type imaging lenses, however, it is difficult to provide back focus relatively long, beside keeping the exit pupil away from the image surface, and providing a very compact design with a wide viewing angle.